Process for the multi-reactor synthesis of zeolite crystals having a controlled particle size

ABSTRACT

The present invention relates to a process for preparing zeolite crystals having a multimodal particle size distribution, and the sizes of which are between 0.02 μm and 20 μm, said process comprising feeding at least two reactors each with a synthesis gel capable of forming zeolite crystals, carrying out a crystallization reaction, in parallel, in each of the at least two reactors, and mixing the reaction media of the at least two reactors, after the start of at least one of the crystallization reactions.

The present invention relates to the field of zeolites, more precisely the field of the industrial synthesis of zeolite crystals and more particularly that of the industrial synthesis of zeolite crystals having a controlled particle size.

The synthesis of zeolite crystals (or more simply “zeolite synthesis” in the remainder of the present document) is carried out conventionally in industry in a large-sized stirred “batch” reactor, generally with heating of the synthesis gel and/or reaction medium by injection of steam and/or by a heating jacket. The preparation of the synthesis gel consists in mixing a sodium aluminate solution with a sodium silicate solution, it being possible for this mixing to be carried out either in equipment upstream of the crystallization reactor or directly in the crystallization reactor.

It is then often necessary to carry out a low-temperature maturing phase, generally at a temperature below 40° C., for a longer or shorter duration, generally varying from a few tens of minutes to a few tens of hours, depending on the type of zeolite structure desired. This maturing phase makes it possible to form seed crystals that will give, by the growth thereof, zeolite crystals after the crystallization phase carried out at higher temperature.

The addition of seeds to the synthesis gel (seeding process) makes it possible however to eliminate this low-temperature maturing phase. Under these conditions, it is thus possible to control the mean size of the crystals by adjusting the amount of seeds introduced into the synthesis gel and thus form a reaction medium capable of forming zeolite crystals.

Therefore it is thus possible, or at the very least theoretically possible, to obtain zeolite crystals of various particle sizes, varying for example from several tens, or even several hundreds of nanometres to several tens of micrometres, it being understood that a synthesis reaction, with operating conditions suitable for this synthesis, results in the formation of zeolite crystals of relatively well-controlled, generally monomodal particle size characterized by a broader or narrower particle size distribution.

However, the fields of use of zeolites are today increasingly varied and increasingly require elaborate technologies, so that it is often necessary to have available zeolites having a monomodal, bimodal or even multimodal controlled particle size, the full width at half maximum (FWHM) of which can be regulated and controlled.

Indeed, it may be necessary these days to be able to provide zeolite crystals of controlled particle size, and more particularly of modulated particle size, in order for example to increase the compactness, the density, etc., depending on the targeted applications. In these applications, it is moreover often required to replace used zeolites, and it is therefore essential to be able to replace these used zeolites with new zeolites having the same particle size characteristics.

Versatile synthesis processes are therefore sought that make it possible to obtain zeolites having bimodal or multimodal particle size distributions that are well controlled and above all modulated, that is to say repeatable over time.

Synthesis processes are known that result in zeolite crystals having a relatively narrow monomodal particle size distribution. However, besides the fact that it is often difficult to reproduce several identical successive syntheses (often “batch” syntheses) very accurately in order to achieve the same particle size characteristics, these techniques do not generally make it possible to obtain multimodal particle size distributions.

In order to obtain zeolite crystals of different and well-defined sizes, it could be envisaged to produce physical mixtures of zeolite crystals having a perfectly well-defined monomodal distribution. Mixtures of zeolite crystals, that is to say physical mixtures of dry powders, are in reality rather unsatisfactory; indeed, it is very difficult to obtain homogeneous mixtures of crystals having particle sizes between several tens of nanometres and several tens of micrometres.

Therefore, there remains a need for a process for preparing zeolite crystals of controlled particle size, of modulated particle size, of monomodal or multimodal particle size distribution, with an adjustable FWHM, and having a particle size between several tens of nanometres and several tens of micrometres.

The Applicant has now discovered that the objectives described above can be achieved in full or at the very least in part by means of the process described below, which forms a first subject matter of the present invention. Other advantages and yet other subjects will appear in the description of the present invention that follows.

The Applicant has now discovered that reproducible and homogeneous (stable throughout the production) multimodal particle size distributions of zeolite crystals can be easily obtained by mixing the reaction media from several synthesis reactors, at the end of or during the crystallization, which produce different crystal sizes, and by adjusting the proportions of the reaction media introduced.

In order to ensure good management of the multimodality of the particle size distribution of zeolite crystals, the process of the present invention comprises:

-   -   the production in parallel, in at least two different synthesis         reactors, of zeolite crystals of different particle size         distributions, then     -   the mixing, after the start of the crystallization reaction,         that is to say during the crystallization reaction or at the end         of the crystallization, of the reaction media (suspensions of         zeolite crystals thus produced) of the at least two different         reactors, in proportions resulting in the desired multimodal         particle size distribution.

Thus, the present invention firstly relates to a process for preparing zeolite crystals having a multimodal particle size distribution, and the sizes of which are between 0.02 μm and 20 μm, said process comprising at least the following steps:

-   a) preparing a synthesis gel by mixing at least one source of     silica, at least one source of alumina and optionally, but     preferably, at least one aqueous alkali metal or alkaline-earth     metal hydroxide solution, -   b) feeding at least two reactors each with a synthesis gel capable     of forming zeolite crystals, -   c) carrying out a crystallization reaction, in parallel, in each of     the at least two reactors, -   d) mixing the reaction media of the at least two reactors, and -   e) filtering the mixture of the reaction media obtained in step d),     in order to separate the crystals produced from the mother liquors.

The synthesis gel may be any type of composition well known to a person skilled in the art depending on the type of zeolite to be prepared and typically comprises at least one source of silica and at least one source of alumina, and/or any other source of element(s) that may constitute a zeolite framework, for example a source of phosphorus, of titanium, of zirconium, etc. It is also possible, or even preferable, to add at least one aqueous solution of an alkali or alkaline-earth metal hydroxide, preferably of an alkali metal hydroxide, typically of sodium hydroxide and/or also organic structure-directing agents or templates.

A source of silica is understood to mean any source well known to a person skilled in the art and in particular a solution, preferably an aqueous solution, of silicate, in particular of alkali or alkaline-earth metal silicate, for example of sodium silicate, or of colloidal silica.

A source of alumina is understood to mean any source of alumina well known to a person skilled in the art and in particular a solution, preferably an aqueous solution, of aluminate, in particular of alkali or alkaline-earth metal aluminate, for example of sodium aluminate.

The concentrations of the various solutions of silica and alumina are adapted depending on the nature of the source of silica, of the source of alumina, the respective proportions of the sources of alumina and of silica to which the alkali or alkaline-earth metal hydroxide solution and/or one or more organic structure-directing agents are added, according to the knowledge of a person skilled in the art. Information will in particular be found on the chemical nature of the organic structure-directing agents to optionally be used as a function of the zeolite to be synthesized on the website of the International Zeolite Association (www.iza-online.org), for example and non-limitingly tetramethylammonium (TMA), tetra-n-propylammonium (TPA), methyltriethylammonium (MTEA).

The respective proportions and concentrations of the various silica and alumina solutions are known to a person skilled in the art or may be easily adapted by a person skilled in the art depending on the nature of the zeolite that it is desired to prepare, from data in the literature.

The synthesis gel from step a) is prepared as described above by mixing sources of silica and alumina in a basic medium. This mixing is advantageously carried out in a rotor-stator shear mixer, that is to say a shear mixer comprising a rotor that rotates at high speed and that makes the mixture pass through a stator, the geometry of which may vary.

The degree of shear is defined by the shear rate γ in s⁻⁻¹ which is equal to the tip speed of the rotor divided by the thickness of the gap between the rotor and the stator. The tip speed V_(p) is calculated from the speed of rotation V_(r) and from the diameter of the rotor d according to the equation: V_(p)=V_(r)πd_(r) (expressed in m·s⁻¹), where V_(r) is the speed of rotation expressed in rev·s⁻¹, d_(r) is the diameter of the rotor (expressed in m) and γ is equal to V_(p)/e, where e represents the distance of the gap between the rotor and the stator (expressed in m).

The shear rate generally applied is between 10 000 s⁻¹ and 200 000 s⁻¹, preferably between 10 000 s⁻¹ and 100 000 s⁻¹.

The at least two reactors are each fed in step b) with a synthesis gel of zeolite crystals by any suitable means for transferring a fluid, for example by gravity flow, by siphoning or by pumping. The monitoring of the flow rates of the synthesis gel at the inlet of each of the at least two reactors and/or of the production of crystals at the outlet of each of the at least two reactors may be obtained according to any means known to a person skilled in the art and preferably by means of pumps, optionally combined with flow regulators.

The at least two reactors are each fed by a synthesis gel of zeolite crystals. The synthesis gels may be identical or different, that is to say that they may be prepared from various silica and alumina solutions introduced in different respective proportions and concentrations that are known to a person skilled in the art or that can be easily adapted by a person skilled in the art depending on the nature of the zeolite that it is desired to prepare, from data in the literature.

One preferred embodiment of the process of the present invention comprises the introduction of one or more seeding agents into the synthesis gel(s) upstream of or inside at least one of the synthesis reactors or in the at least two synthesis reactors. A seeding agent is understood to mean a solution or a suspension, in liquid form or in the form of a gel, a solid or a liquid that promotes the orientation of the synthesis towards the desired zeolite. Such solids and liquids that promote the orientation of the synthesis towards the desired zeolite are well known to a person skilled in the art and are for example chosen from nucleating gels, zeolite crystals, mineral particles of any nature, etc., and also mixtures thereof.

According to a preferred aspect, the seeding agent is a nucleating gel, and, more preferably, said nucleating gel comprises a homogeneous mixture of a source of silica (for example sodium silicate), a source of alumina (for example alumina trihydrate), optionally but advantageously a strong mineral base, for instance sodium hydroxide, potassium hydroxide or calcium hydroxide to mention but the main ones and the ones most commonly used, and water. One or more structure-directing agents, typically organic structure-directing agents, may also optionally be introduced into the nucleating gel.

The seeding agent(s) may be mixed with the synthesis gel according to any technique well known to a person skilled in the art and preferably using a static mixer, which has the advantage of promoting the homogenization of said mixture.

The reactors used may be of any type well known to a person skilled in the art and suitable for the type of synthesis envisaged, for example stirred reactor for batch-mode syntheses and tubular reactor for continuous-mode syntheses. In the process of the invention generally 2 or more reactors, of identical or different types, preferably 2, 3, 4 or 5 reactors of identical or different types, more preferably 2 or 3 reactors of identical or different types, more preferably 2 reactors of identical or different types, typically two reactors of identical types, are present.

It is moreover preferred to conduct the process continuously, and in this case use will preferably be made of 2, 3 or 4 tubular reactors, more preferably 2 or 3 tubular reactors, typically 2 tubular reactors.

A “stirred reactor” is understood to mean a reactor equipped with a stirring system, typically equipped with one or more agitators mounted on the same shaft or on different shafts, for example and non-limitingly, with blade agitator(s), blender(s), or else an Archimedes screw type mixer and optionally equipped with one or more systems of baffles or deflectors.

A “tubular reactor” is understood to mean a reactor or system of reactors having length-to-diameter (or equivalent diameter) ratios of greater than 3, preferably of greater than 10 and more preferably greater than 50, and defining a crystallization reaction zone subjected at least in part to stirring means, whether these are stirring spindles, passive systems such as baffles, restrictions, rings or deflectors or an oscillating or pulsating system (enabling a back-and-forth movement of the reaction medium to be generated by means for example of a piston, membrane), etc., and also two or more of these techniques combined.

In a preferred embodiment of the invention, the tubular reactor is provided with restrictions and equipped with a system that enables pulses to be imparted to the fluid circulating in the reactor, as for example described in application US 2009/0304890 by NiTech.

Step c) of carrying out the crystallization reactions is conducted according to methods known to a person skilled in the art, that is to say either by maturing/crystallization starting from a synthesis gel, or by direct crystallization after seeding by various types of seeds. The seeded process is preferred in that it enables a better management of the size of the crystals produced.

The crystallization reactions are carried out in parallel, each in a reactor, and may be carried out simultaneously and/or sequentially and/or successively, preferably simultaneously.

The crystallization reaction is generally carried out at high temperature, that is to say at a temperature between 60° C. and 200° C., preferably between 80° C. and 160° C. The crystallization of the synthesis gel usually occurs spontaneously in the reactor and is promoted by the seeding agent(s) added to said synthesis gel. The crystallization is also promoted by the temperature applied to the synthesis gel, but also by any static or dynamic means of agitating the synthesis gel within the reactor as explained above.

The term “promoted” is understood to mean a better initiation of crystallization and/or greater crystallization kinetics.

The crystallization reaction may be carried out under pressure, for example under autogenous pressure, under atmospheric pressure, or more generally under any pressure, typically between atmospheric pressure and 1.5 MPa.

The reaction media of the at least two reactors operating in parallel are then mixed at any moment, as soon as the crystallization reaction has begun in one of the at least two reactors, preferably in the at least two reactors, this being in the proportion that enables the desired multimodal distribution to be obtained.

The crystallization reaction is considered to have begun in the reactor as soon as the degree of crystallinity, analyzed by x-ray diffraction (XRD) on a sample withdrawn from said reactor then dried at 80° C. for 4 hours, is greater than 5%, preferably greater than 10%, more preferably greater than 30% and very preferably greater than 50%.

When the syntheses are carried out in continuous mode, for each of the reactors, which are preferably tubular reactors, the synthesis gel is preferably prepared continuously using a shear mixer operating continuously. It is then possible to introduce into the synthesis gel, by any suitable means described above and continuously, one or more seeding agents in order to seed and adjust the size of the crystals that will be obtained after the crystallization.

It may also be envisaged to operate with seeding agents and/or synthesis gels of different natures for the purpose of obtaining zeolite crystals that are themselves of different structures. The process of the present invention may specifically be implemented in order to synthesize zeolite crystals of different structures.

Several reactors (at least two) operate in parallel, at suitable flow rates, in order to adjust the proportion of the various size categories of the zeolite crystals. The reaction media produced continuously in these reactors and that contain the zeolite crystals are mixed continuously as soon as the crystallization reaction has begun in one of the at least two reactors, preferably in the at least two reactors, this being in the proportion that enables the desired multimodal distribution to be obtained.

According to a preferred embodiment, the process of the present invention is carried out in continuous mode. Unlike batch mode, continuous mode offers the advantage of facilitating the adjustment of the multimodal distribution by controlling the flow rates of each tubular reactor.

The process of the present invention is characterized by the fact that at least two syntheses of zeolite crystals are carried out in parallel, the reaction media of which are combined as soon as the crystallization reaction has begun in one of the at least two reactors, preferably in the at least two reactors, this being in the proportion that enables the desired multimodal distribution to be obtained.

The mixing of the reaction media may be carried out according to any method well known to a person skilled in the art, the methods that are particularly suitable are those that enable an effective and homogeneous mixing of aqueous media in which solid particles are in suspension. Among the methods that can be used, mention may be made, as nonlimiting examples, of those using a static mixer, or any other type of blade mixer, propeller mixer, multiple paddle mixer, or else a simple system of pipes that join up into a single pipe (for example “Y” tube in the case of two reactors). It is also possible to combine one or more mixing techniques.

The fact of mixing reaction media containing the zeolite crystals in the form of liquid suspensions makes it possible to obtain, after filtration/washing and drying, a mixture of zeolite crystals that is homogeneous in terms of particle size distribution. This result cannot be obtained easily, that is to say economically and with good reproducibility, when dried zeolite crystals of different particle sizes are mixed. Specifically, in this case, segregation phenomena are often observed that are inherent to the dry mixing of zeolite crystals of various sizes, therefore of various individual masses.

The reaction media resulting from the crystallization step c) are different in that they contain zeolite crystals that are different, either in terms of size, or in terms of nature, or in terms of nature and size. It is however preferred to use the process of the invention for the synthesis of a single crystalline form of zeolite, with however different particle size distributions, so as to obtain zeolite crystals having a bimodal or multimodal particle size distribution.

A person skilled in the art easily understands the great flexibility imparted by the process of the invention, enabling populations of zeolite crystals having controlled and modulated particle size distributions to be generated.

For example, the respective flow rates and/or the respective amounts of each of the reaction media may be adjusted in order to regulate the proportions of each of the reaction media entering the mixing step d) and thus to easily control the desired particle size distribution of the mixture of zeolite crystals at the end of the process of the present invention.

The process of the invention thus makes it possible to obtain a multimodal size distribution of crystals that is adjustable and modulated by mixing several reaction media, each of them being derived from a synthesis, the parameters of which impose a well determined and modulated particle size.

By means of the process of the invention, it is now possible to prepare zeolite crystals industrially, advantageously continuously, by finely adjusting the number-size distribution of the zeolite crystals. It is thus possible to obtain an easy, effective and economical industrial production of zeolite crystals with a multimodal particle size distribution that is adjustable and modulated, that is to say reproducible and stable over time.

In one preferred embodiment, the process of the invention is a process for the continuous synthesis of a mixture of zeolite crystals having a bimodal particle size distribution, said process being carried out in two tubular reactors operating in parallel with different synthesis conditions so as to produce crystals of different particle size, the outlets of the two reactors being joined together by means of Y-shaped tubing.

According to a preferred aspect of the present invention, the reactions for the syntheses of the zeolite crystals are carried out in the presence of one or more seeding agents, as defined above.

At the end of the mixing step d), the mixture of said reaction media is filtered in order to separate the crystals produced on the one hand and the mother liquors on the other hand. This filtration may be carried out according to any method well known to a person skilled in the art, and for example by one or more methods chosen from centrifugation, filter press filtration, belt filter filtration, rotary filter filtration, etc.

The crystals obtained at the end of step e) may optionally be subjected to one or more conventional treatments well known to a person skilled in the art, such as washing, cation exchange, drying, impregnation, activation, etc., it being possible for this or these treatment(s) to be carried out in batch mode or continuously, advantageously continuously. For example, the crystals obtained may be subjected to one or more washings with water, so as to eliminate the residual mother liquors that might still be present.

The crystals obtained may also be dried, according to the conventional techniques for drying zeolite crystals, for example at temperatures between 40° C. and 150° C., for a duration that may vary between several minutes and several hours, typically between several minutes and 10 hours. The drying operation at a temperature below 40° C. could prove to be much longer and thus economically unprofitable, whereas a drying temperature above 150° C. could result in a greater or lesser deterioration of the still wet zeolite crystals.

After drying, the zeolite crystals may be used as is, but they are advantageously activated, here too according to conventional activation techniques well known to a person skilled in the art, for example at temperatures between 150° C. and 800° C., for a duration that varies from several minutes to several hours, and typically from several minutes to 10 hours.

The mother liquors resulting from the filtration step e) may advantageously be recycled. One of the advantages of this recycling is to thus enable the reduction in the consumption of sodium hydroxide by introducing the mother liquors directly into the reaction medium of one of the at least two reactors or into the silicate solution or else into the aluminate solution (typically that are respectively the sources of silica and of alumina in step a) of the process), or else into the synthesis gel, but may also enable a substantial reduction in energy consumption. Before being recycled, the mother liquors may optionally have undergone one or more treatments chosen from ultrafiltration, reconcentration, distillation, etc.

The process of the present invention is very advantageously carried out continuously, preferably completely continuously, that is to say without a step in batch mode.

Thus, according to one embodiment of the process of the present invention, each reactor delivers a given and advantageously narrow particle size fraction determined by the quality and the amount of seeding agent(s) introduced into the synthesis gel. The total amount of seeding agent(s) added to the process of the present invention represents between 0.005% and 10% by weight relative to the synthesis gel, preferably between 0.01% and 5% and more preferably between 0.01% and 3% by weight relative to the synthesis gel introduced at the start into each reactor. Subsequently, the various reaction media are mixed to obtain a multimodal size distribution which is the sum of the various monomodal fractions produced in each of the synthesis reactors.

Thus, the process of the present invention enables the synthesis, advantageously continuously, of zeolite crystals having a multimodal particle size distribution, this being in a manner that is homogeneous and reproducible, and stable over time.

The determination of the particle size distribution corresponds here to the particle number-size distribution of the diameter of the zeolite crystals. This determination is carried out from images obtained by observation under a scanning electron microscope (SEM). For this, a set of images is taken at a magnification of at least 3000. All of the crystals present on the images are measured using dedicated software, for example the Smile View software published by LoGraMi, so as to measure at least 300 crystals, then the distribution by number is plotted in the form of a histogram with categories adapted to the particle size of the crystals, for example categories every 0.2 μm for counting micrometric crystals or for example categories every 0.02 μm for counting crystals of several tens of nanometres.

A “multimodal particle size distribution” is understood to mean a distribution of multimodal size, that is to say having at least two “separate” peaks, in other words at least two “resolved” peaks. The value of the diameter at the top of the peak is referred to as “mode” or else “dominant value”, and represents the most common value of the peak. When a distribution has two separate (or resolved) peaks, the distribution is said to be bimodal.

The concept of multimodality is defined using a “criterion” R, known as the “resolution factor” which characterizes the separation or non-superimposition of the peaks.

The various peaks are compared to a Gaussian, characterized by its mode d and its full width at half maximum 6, from which it is possible to deduce the width of the base of the peak ω=1.7 δ.

The resolution factor R of 2 adjacent peaks A and B is conventionally defined (see for example: “Notions fondamentales de chromatographie” [Fundamental chromatography concepts] by Marie-Paule Bassez: http://chemphys.u-strasbg.frimpb/teach/chromato1/img0.html) using the following equation:

R=2(dB−dA)/(ωB+ωA),

where dA and dB are respectively the modes of the peaks A and B (in μm), and ωA and ωB are respectively the widths of the base of the peaks A and B (in μm).

As a general rule, two peaks are considered to be resolved, or else completely separate, when the value of R is greater than 1.5. In the context of the present invention, a particle size distribution has a difference in modality when the resolution factor R is greater than 0.5. In the present description, the particle size distribution is considered to be multimodal since at least two peaks are resolved. When the particle size distribution comprises only two resolved peaks, reference is then made to a bimodal particle size distribution.

The process according to the present invention thus enables the production of zeolites, the crystals of which have a controlled or even modulated, bimodal or even multimodal, particle size distribution, it being possible for this production to be carried out industrially very easily, thus enabling a production of large amounts of such zeolites of controlled or even modulated particle size, with production costs much lower than those observed, for example, with productions according to the conventional processes known today.

The zeolites that can be prepared according to the process of the present invention may be of any type and, for example and non-limitingly, any zeolite of MFI type, and in particular silicalite, any zeolite of MOR type, of OFF type, of MAZ type, of CHA type and of HEU type, any zeolite of FAU type, and in particular zeolite Y, zeolite X, zeolite MSX, zeolite LSX, any zeolite of EMT type or else any zeolite of LTA type, i.e. zeolite A, and also the other zeotypes, such as for example titanosilicalites.

The term “zeolite MSX” (medium silica X) means a zeolite of FAU type with an Si/Al atomic ratio of between about 1.05 and about 1.15, limits included. The term “zeolite LSX” (low silica X) means a zeolite of FAU type with an Si/Al atomic ratio equal to about 1.

The process according to the invention is particularly suitable for the preparation of zeolites chosen from zeolites of MFI type, and in particular silicalite, of FAU type, and in particular zeolite Y, zeolite X, zeolite MSX, zeolite LSX, and of LTA type, i.e. zeolite A, and also the zeolites of CHA type and the zeolites of HEU type.

The process according to the invention is moreover very particularly suitable for the preparation of any zeolite of FAU type, and in particular zeolite X, zeolite MSX, zeolite LSX. The zeolites of MFI type, and in particular silicalite, may also be very advantageously prepared according to the process of the invention.

Moreover, the continuous preparation process of the present invention is not limited to the preparation of the zeolites described above, but also includes the corresponding zeolites with hierarchical porosity. The zeolites with hierarchical porosity are solids comprising a microporous network linked to a network of mesopores, and thus make it possible to reconcile the properties of accessibility to the active sites of the mesoporous zeolites known from the prior art and those of maximum crystallinity and maximum microporosity of “conventional” zeolites (without mesoporosity). In this case, specific structure-directing agents are introduced into the reaction medium of step a), for example structure-directing agents of organosilane type as described in document FR 1 357 762.

The synthesis process of the present invention consequently enables an easy and economical industrial synthesis of zeolite crystals, the at least bimodal particle size distribution of which is homogeneous, controlled, or even modulated. Moreover, it has been observed that the process according to the invention is very stable over time when it is carried out in continuous mode. These zeolite crystals find very interesting uses in many fields of application.

Specifically, owing to the process of the invention, it is now possible to more easily obtain zeolite crystals of multimodal distribution in a modulated and homogeneous manner, unlike what would be obtained with mixtures of crystals of various particle sizes.

The multimodal particle size distribution of the zeolite crystals obtained by means of the process of the invention makes it possible to obtain crystals having especially a bulk density that is high, and in particular higher compared to crystals of monomodal particle size distribution. Specifically, the small crystals can be considered to occupy the spaces between the large crystals.

This high bulk density of the zeolite crystals obtained with the process of the invention makes it possible to obtain a very distinctive adsorption performance, especially in terms of adsorption volume capacity.

The zeolite crystals obtained with the process of the invention thus find very interesting applications in the field of adsorption, separation and purification of gases and liquids. As nonlimiting examples, the zeolite crystals obtained according to the process of the present invention may advantageously be used as adsorbent fillers in polymer-based composites, as a constituent of agglomerated zeolite adsorbents used in adsorption separation or purification processes such as pressure swing and/or temperature swing processes, or else in chromatographic type separation processes (fixed beds, moving beds, simulated moving beds), in applications as varied as the purification of industrial gases, the separation of nitrogen and oxygen, the purification of natural gas or of synthesis gas, or else the purification of various petrochemical fractions, the separation of isomers in refining, etc.

The degree of crystallinity, and also the purity of the zeolite synthesized, are evaluated by x-ray diffraction analysis, a technique known to a person skilled in the art under the acronym XRD. This identification is for example carried out on a Bruker XRD apparatus.

This analysis makes it possible not only to determine the amount of crystalline phase(s) present, but also to identify and quantify the possible different zeolites present, each of the zeolites having a unique diffractogram defined by the positioning of the diffraction peaks and by the relative intensities thereof. The noncrystalline phases are not detected by the x-ray diffraction analysis.

This analysis is also used to determine the degree of crystallinity of the reaction medium in order to evaluate whether the crystallization reaction has begun. In this case, a sample of the reaction medium is withdrawn, dried at 80° C. for 4 hours, then analysed by XRD.

The zeolite crystals (or the dried samples) are ground and then spread and levelled out on a sample holder by simple mechanical compression. The conditions under which the diffractogram is acquired on the Bruker D5000 machine are as follows:

-   Cu tube used at 40 kV-30 mA; -   slit size (divergent, scattering and analysis slits) =0.6 mm; -   filter: Ni; -   sample device rotating at: 15 rpm; -   measuring range: 3°<2θ<50°; -   increment: 0.02°; -   counting time per increment: 2 seconds.

Interpretation of the diffractogram obtained is performed with the EVA software with identification of the zeolites using the ICDD PDF-2 release 2011 database.

The amount of crystals, by weight, is determined by XRD analysis; this method is also used to measure the amount of noncrystalline phases. This analysis is performed on a Bruker machine, and the amount by weight of the zeolite crystals is then evaluated using the TOPAS software from the company Bruker.

The crystallinity (or degree of crystallinity) corresponds to the ratio of the sum of the weight fractions of the crystalline phases present, relative to the total weight of the sample.

The purity is expressed as a weight percentage of desired crystalline phase relative to the total weight of the sample. 

1. Process for preparing zeolite crystals having a multimodal particle size distribution, and the sizes of which are between 0.02 μm and 20 μm, said process comprising at least the following steps: a) preparing a synthesis gel by mixing at least one source of silica, at least one source of alumina and optionally, at least one aqueous alkali or alkaline-earth metal hydroxide solution, b) feeding at least two reactors each with a synthesis gel capable of forming zeolite crystals, c) carrying out a crystallization reaction, in parallel, in each of the at least two reactors, d) mixing the reaction media of the at least two reactors, and e) filtering the mixture of the reaction media obtained in step d), in order to separate the crystals produced from the mother liquors.
 2. Process according to claim 1, wherein the at least two reactors are each fed by a synthesis gel of zeolite crystals, it being possible for the synthesis gels to be identical or different.
 3. Process according to claim 1, wherein one or more seeding agents are introduced into the synthesis gel(s) upstream of or inside at least one of the synthesis reactors or in the at least two synthesis reactors.
 4. Process according to claim 1, wherein the seeding agent is chosen from nucleating gels, zeolite crystals, mineral particles, and mixtures thereof.
 5. Process according to claim 1, wherein the reactors used are stirred reactors for syntheses in batch mode and tubular reactors for syntheses in continuous mode.
 6. Process according to claim 1, wherein 2 or more reactors, of identical or different types, are present.
 7. Process according to claim 1, conducted continuously, in 2, 3 or 4 tubular reactors
 8. Process according to claim 1, wherein the crystallization reaction is carried out at a temperature of between 60° C. and 200° C.
 9. Process according to claim 1, wherein the crystallization reaction is carried out under a pressure between atmospheric pressure and 1.5 MPa.
 10. Process according to claim 1, wherein the reaction media of the at least two reactors operating in parallel are mixed at any moment, as soon as the crystallization reaction has begun in one of the at least two reactors.
 11. Process according to claim 1, wherein the mother liquors from the filtration step e) are recycled.
 12. Process according to claim 1, wherein the zeolite crystals are crystals of zeolite chosen from zeolites of MFI type, zeolites of MOR type, zeolites of OFF type, zeolites of MAZ type, zeolites of CHA type, zeolites of HEU type, zeolites of FAU type, zeolites of EMT type, zeolites of LTA type, and other zeotypes. 